[1] Natl. Inst. for Res. in Inorg. Mat., 1-1 Namiki, Tsukuba, Ibaraki 305, Japan
[2] Institute of Applied Physics, University of Tsukuba, Tsukuba, Ibaraki 305, Japan
[3] Sony Corporation Research Center, 174 Fujitsuka-cho, Hodogaya-Ku, Yokohama 240, Japan